1. Field of the Invention
The present invention relates to marking materials and, more particularly, is directed to phosphorescent marking materials and a method for suspending pigment particles.
2. Description of the Prior Art
Phosphorescent marking crayons currently available on the market have met with limited success because they frequently fail to exhibit favorable glow and marking characteristics. Generally, the cause of these deficiencies relates to the suspension characteristics of the phosphorescent pigment particles in the crayon. Although various techniques have been suggested to assist in the suspension of pigment particles in marking crayons, the materials used in these methodologies can adversely impact transparency, mechanical strength, and marking ability.
Attempts to produce markers having good legibility and transparency to phosphorescent light have focused on the use of additives in conjunction with pigmenting materials. For example, U.S. Pat. No. 5,116,533, which is owned by the assignee of the present application, teaches suspension of pigment particles in a phosphorescent marker by means of filler particles. It has been found that, to a certain degree, these filler particles decrease transparency and interfere with the transmission of light to and from phosphorescent particles.
Alternative techniques presented in the prior art have utilized viscosity modifiers at selected concentrations. Two types of such modifiers which are typically used are long flow viscosity modifiers and short flow viscosity modifiers. Typical long flow viscosity modifiers are polyethylene and vinyl acetates. Salt activated waxes can be used as a short flow viscosity modifiers. The viscosity modifier concentrations taught in the prior art frequently produce undesirable effects during the manufacturing process such as agglomeration of particulates or rapid gelling of the marking material.
A need has arisen for an improved suspension medium and, in particular, for a marking material which exhibits improved transparency, mechanical strength, and marking characteristics.